Natural History Highlight

UV Rays Shed New Light on the Hope Diamond’s Mysterious Red Glow

By Amanda Thornburg

The 45.52 carat Hope Diamond is in a platinum setting surrounded by sixteen white pear-shaped and cushion-cut diamonds designed by Pierre Cartier in about 1910. Photograph by Chip Clark.

Hundreds of rare precious gemstones are on display in the Gems and Minerals Galleries at the Smithsonian’s National Museum of Natural History. According to Dr. Jeff Post, curator of the United States National Gem and Mineral Collection and avid mineralogist, few of those gems garner more attention than the world famous and Smithsonian’s own, Hope Diamond. With its breathtaking beauty and mysterious past, the Hope Diamond intrigues millions of museum visitors each year; but beyond its rumored curse, the world’s largest blue diamond is proving to be a unique scientific specimen.

The 45.52-carat blue diamond puzzles scientists because of the fiery red glow it gives off for several minutes after being exposed to ultra-violet light. Scientists refer to this phenomenon as phosphorescence. “It looks like a glowing orange coal in your barbeque grill,” explains Post. “It has been described as one of the unique properties of this unique diamond, something special to the Hope Diamond.” No comprehensive studies on the nature of the phosphorescence exist, which has made Dr. Jeff Post question the impressive glow for years. “There didn’t seem to be a lot of consistency, or certainly no quantification of the nature of the phosphorescence,” Post says. Thus, he and a team of researchers took on the challenge to dispel the deep dark secrets of the Hope Diamond.

In a curious effort, Post and colleagues from the U.S. Naval Research Laboratory, Ocean Optics Instrument Company, and Penn State University eagerly snagged the Hope Diamond from its glass enclosure, along with the world’s second largest deep-blue diamond, the Blue Heart Diamond, and blue diamonds from the Aurora Butterfly of Peace, a temporary collection of 240 colored gemstones. They hand carried the gems to the Smithsonian’s highly secure blue room vault, where hundreds of the museum’s most expensive and rare gems are located. Using a portable instrument that measures wavelengths of light, known as a spectrometer, the researchers exposed each diamond to ultra-violet light in order to measure the intensity of light given off, and the rate at which it faded. As reported in the January 2008 issue of the journal Geology, the researchers developed a better understanding of phosphorescence behavior, and to their pleasant surprise, discovered a way to essentially “fingerprint” blue diamonds.

The intense orange phosphorescence of the Hope Diamond is only visible in a dark room after exposure to ultraviolet light. One of the diamonds surrounding the Hope is phosphorescing blue. Photograph by John Nels Hatleberg.

Post and his team of researchers concluded that red phosphorescence is not just specific to the Hope Diamond, but indeed a property of all natural blue diamonds. Trace impurities of the element boron give rise to a diamond’s deep blue color. Presumably, the boron interacts with trace amounts of nitrogen to give each diamond its unique phosphorescence behavior. Hoping to see a trend among the diamonds tested, researchers found just the opposite. “The plot just scattered, indicating that each of these diamonds had its own set of these characteristics,” said Post. “That gave us a way of fingerprinting a particular blue diamond.” In addition, the researchers tested synthetic diamonds doped with boron and natural heat-treated blue diamonds. The artificially treated blue diamonds had a completely different phosphorescence spectrum than the natural blue diamonds, which could be useful to gemologists when identifying the real from the fake.

Dr. Post’s passion and natural affinity for crystals inspires him to probe for new questions regarding the Hope Diamond. “There is always more to learn, and as new ideas, new techniques, new questions come up, we will continue to learn from it,” says Post, “Usually, one study raises as many questions as it answers, and so it always opens up new lines of potential research that will hopefully lead to a more in-depth understanding of the diamond itself.” This study has also been a nice change for Dr. Post who is used to studying materials that most people have never even heard of. “Much of my day to day research is on mud and muck and clays that are critically interesting to our environment, but yet there is no aesthetic pleasure whatsoever,” he says. “Then on the other side of the research coin is working on things like the Hope Diamond, so it’s an interesting stretch, but also a fun balance to have.”